Protective compositions and methods of making same

ABSTRACT

The present invention is directed to compositions derived from polymers containing metal-nitrogen bonds, which compositions exhibit, among other things, desirable oxidation resistance, corrosion resistance and hydrolytic stability when exposed to adverse environments, whether at ambient or at elevated temperatures, and which may be useful as, for example, protective coatings on surfaces.

FIELD OF THE INVENTION

The present invention relates to novel processes for formingcompositions which may be utilized, among other things, as protectivecoatings on surfaces. Particularly, the present invention is directed tocompositions derived from polymers containing metal-nitrogen bonds,which compositions exhibit, among other things, desirable oxidationresistance, corrosion resistance and hydrolytic stability when exposedto adverse environments, whether at ambient or elevated temperatures,and which may be useful as, for example, protective coatings onsurfaces.

BACKGROUND OF THE INVENTION

Significant efforts have been directed to modifying the properties ofknown or existing materials in a manner which renders the materialssuitable for use in environments which normally would adversely affectsuch materials. For example, one such modifying approach generallyrelates to coating onto a surface of a substrate material a secondmaterial having properties which differ from the underlying substratematerial.

Various methods exist for coating substrate materials. A first categoryof coating processes is generally referred to as overlay coatings.Overlay coatings involve, typically, a physical deposition of a coatingmaterial onto a substrate. The coating material typically enhances theperformance of the substrate by, for example, increasing the erosionresistance, corrosion resistance, high temperature strength, etc., ofthe substrate material. These overlay coatings typically extend the lifeof the substrate material and/or permit the use of the substratematerial in a number of environments which normally might adverselyaffect and/or destroy the utility of the substrate material absent theplacement of the overlay coating thereon.

Commonly utilized overlay coating methods include Painting, Dipping,Spraying, Spin Coating, Chemical Vapor Deposition, Hot Spraying,Physical Vapor Deposition, etc. Such methods as Painting, Dipping,Spraying and Spin Coating are readily understood to an artisan ofordinary skill in the art as widely-applicable conventional coatingtechniques. Chemical Vapor Deposition utilizes a chemical process whichoccurs between gaseous compounds when such compounds are heated.Chemical Vapor Deposition will occur so long as the chemical reactionproduces a solid material which is the product of the reaction betweenthe gaseous compounds. The Chemical Vapor Deposition process istypically carried out in a reaction chamber into which both a reactivegas and a carrier gas are introduced. A substrate material is placedinto contact with the reactant and carrier gases so that reactionbetween the gases and deposition of the reaction solid will occur on thesurface of the substrate. Chemical Vapor Deposition processes typicallyinvolve the use of corrosive gases (e.g., chlorides, fluorides, etc.) inthe reaction chamber which can be quite corrosive and must be handledcarefully. Accordingly, even though Chemical Vapor Deposition processesmay produce desirable coatings on some materials, the equipment that isutilized typically is complicated in design and expensive to operate.

Hot Spraying techniques also exist for the placement of an overlaycoating on a substrate material. The three most widely utilized HotSpraying techniques include Flame Spraying, Plasma Spraying, andDetonation Coating.

Flame Spraying utilizes a fine powder which is contained in a gaseousstream and which is passed through a combustion flame to render the finepowder molten. The molten powder is then caused to impinge on a surfaceof a substrate material which is to be coated, which material istypically cold relative to the flame spray. Bonding of the coating offlame-sprayed material to the substrate is primarily of a mechanicalnature. The flame-sprayed coating is usually not fully dense and, thus,is often subsequently treated by a fusing operation to densify thecoating.

Plasma Spraying is somewhat similar to Flame Spraying, except that thefine powder, instead of being passed through an intense combustionflame, is passed through an electrical plasma which is produced by a lowvoltage, high current electrical discharge. As a result, disassociationand ionization of gases occur which results in a high temperatureplasma. The high temperature plasma is directed toward a substratematerial resulting in the deposition of a layer of coating material onthe substrate.

Detonation Coating is a process which has some similarities to FlameSpraying, except that a desired amount of powder is directed at highvelocity (e.g., about 800 meters per second) toward the surface of asubstrate material which is to be coated. While the particles are beingaccelerated in a hot gas stream, the particles melt. Moreover, the highkinetic energy of the particles when impinging on the surface of asubstrate material results in additional heat being generated, therebyassisting the coating process.

Physical Vapor Deposition coatings include, for example, Ion Sputtering,Ion Plating, and Thermal Evaporation.

In Ion Sputtering, a vacuum chamber houses a cathode electrode such thatthe cathode electrode emits atoms and atomic clusters toward a substratematerial to result in a sputtered film or coating being deposited on thesubstrate.

Ion Plating of a substrate material involves the use of a heated metalsource which emits metal atoms toward a substrate material which is tobe coated. Specifically, an electron beam is typically utilized toexcite the metal atoms from the metal source. The excited metal atomsare then directed toward the substrate material to be coated.

Thermal Evaporation also relies on the excitation of atoms from a metalsource. Specifically, in a vacuum chamber, a metal source is heated sothat metal atoms evaporate from the metal source and are directed towarda substrate material to be coated. The metal atoms then collect as acoating on the substrate.

A second general category of coating formation techniques is known asconversion coating techniques. In conversion coating techniques, asubstrate material, typically, is involved in a chemical reaction whichmodifies the composition and/or microstructure of the surface of thesubstrate. These conversion coating techniques also can result indesirable surface morphology modification of substrate materials.Typical examples of conversion coating techniques include PackCementation and Slurry Cementation. Exemplary of specific conversioncoating compositions which may be applied to substrates are conversioncoating techniques referred to as chromating and aluminizing, whereby acoating composition comprising such materials as chromium or aluminum isapplied to a surface of a substrate and reacted with the substrate upon,for example, heating, etc.

Pack Cementation and Slurry Cementation utilize diffusion of one or morematerials to form a surface coating. Specifically, in each of theseprocesses, a substrate material is contacted with a metal sourcematerial such that a metal from the metal source material may diffuseinto the substrate material and/or a component of the substrate materialmay diffuse toward the metal source material. Specifically, for example,in Pack Cementation, a substrate material is buried within a powdermixture which comprises, typically, both a metal which is to react withthe substrate material and an inert material. A carrier gas is theninduced to flow into the powder mixture so that the carrier gas cancarry metal atoms from the metal powder to the surface of the substrateand deposit the metal atoms thereon. In Slurry Cementation, acomposition typically is coated onto a surface of a substrate materialprior to conducting the diffusion process. Both Pack Cementation andSlurry Cementation typically occur in a retort or vacuum furnace atelevated temperatures, and the carrier gas is free to transport metalatoms from the metal powder to the surface of the substrate material.Typical carrier gases include the halogen gases. Many differentapproaches to Pack Cementation have been made; however, most of theseapproaches utilize the above-discussed steps.

Conversion coatings techniques have also been carried out utilizingstarting materials other than the materials discussed above with respectto Pack Cementation and Slurry Cementation. Materials such as organicresins and polymers have also been demonstrated to provide effectivecoatings against, for example, oxidation and corrosion under specifiedenvironmental conditions.

Protective ceramic coatings on, for example, carbon/carbon composites,graphite, carbon fibers and other oxidizable materials, formed frompreceramic polymers which can be converted to ceramic upon heating havebeen described. U.S. Pat. No. 5,198,488 (Patent '488), in the name ofLeonard M. Niebylski, is directed to the preparation of preceramiccompositions which may be used to provide, among other applications,oxidation-resistant coatings on carbon/carbon composites, graphite,carbon fibers and other normally oxidizable materials by dispersingabout 0.1-4 parts by weight of silicon boride in one part by weight of apolysilazane in solution in an organic solvent. The preceramiccompositions are coated onto the oxidizable materials and heated totemperatures of about 675°-900° C. to pyrolyze the preceramiccompositions to ceramic coatings. Patent '488 also teaches that for hightemperature (i.e., higher than 800° C.) oxidation protection, thepyrolysis step is followed by thermal treatment of the coated substrateat about 1075°-1250° C. in an atmosphere containing not more than aminor amount of oxygen.

U.S. Pat. No. 5,196,059 (Patent '059), also in the name of Niebylski, isdirected to preceramic compositions utilized, among other applications,to provide heat, abrasion and oxidation resistant ceramic coatings,prepared by dispersing about 0-3 parts by weight of aluminum-siliconeutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5parts by weight of silicon boride, and about 0.4-5 parts by weight ofsilicon metal in a solution of one part by weight of a polysilazane inan organic solvent. The formation of ceramic coatings in Patent '059 issubstantially as set forth above with respect to Patent '488.

U.S. Pat. No. 5,194,338 (Patent '338), also in the name of Niebylski, isdirected to preceramic compositions utilized, among other applications,to provide protective ceramic coatings on normally oxidizable materialsprepared by dispersing about 0.4-3.0 parts by weight of a ceramic powderselected from silicon carbide, silicon nitride, and mixtures thereof,with one another and/or with up to about 90 percent by weight of (1) ametal boride or (2) a mixture of a metal boride and zirconium metal inone part by weight of a polysilazane. The formation of ceramic coatingsin Patent '338 is substantially as set forth above with respect toPatent '488.

U.S. Pat. No. 5,258,224, in the names of Conrad J. Langlois, Jr., etal., is directed to preceramic compositions which are useful to provideprotective ceramic coatings on normally oxidizable materials and whichcoating compositions are obtained by dispersing solid particlescomprising aluminum nitride particles in an organic solvent solution ofa polysilazane. Ceramic coatings derived from such dispersions may serveas intermediate strata in multilayer ceramic coatings over substrates,such as carbon/carbon substrates, to further protect such substrates,even when exposed to humidity.

The above-discussed coating compositions and techniques have beenbriefly addressed herein to give the reader a general understanding ofthe art. However, it should be understood that specific variations tothe above-discussed compositions and techniques exist. Specifically,each of the coating compositions and/or processes discussed above arediscussed in detail in readily available sources, including textbooks,conference proceedings, and patents. For further information relating tothe detail of these processes, the reader is encouraged to consult theliterature referred to above. However, even from the brief discussionsabove, it should be clear that each of the techniques suffers fromvarious limitations. For example, in the overlay coating techniques, thephysical deposition of a coating onto a substrate material does notinsure an acceptable interface between the substrate and the coating.Specifically, because most of the overlay coating techniques simply relyon the use of a physical bonding between the coating and the substrate,the coating may not adhere adequately to the substrate. Accordingly, thepurpose of the coating may be compromised completely. Additionally, theoverlay coating processes typically depend on the use of somewhatcomplex deposition equipment. For example, Chemical Vapor Depositionrequires the use of relatively complicated control means for controllingthe rate of flow of reactive and carrier gases in a reaction chamber,the ability to handle corrosive gases (e.g., fluorides, chlorides), etc.

Moreover, with regard to the so-called conversion coating techniqueswhich are formed by, for example, Pack Cementation and SlurryCementation techniques, the coatings achieved on substrate materials maynot be uniform due to the inclusion of solid materials or porosity whichresult from exposure of the substrate to either of or both of the powdermetal source and/or inert materials utilized in the Pack Cementation orSlurry Cementation processes. Still further, many of the PackCementation and Slurry Cementation techniques may require the use ofsomewhat complex equipment.

Further, with regard to the techniques discussed in connection with thepreceramic compositions of Niebylski, it is noted that these techniquesinclude the use of specific controlled atmospheres to obtain desirableoxidation resistance at high temperatures, thus requiring the use ofsomewhat complex equipment. Moreover, Langlois, Jr., et al. teaches theuse of specified intermediate strata compositions in multilayer ceramiccoatings to enhance protection of substrates. The use of suchmulticompositional layers within coatings increases not only the cost ofmaking such coatings, but also introduces the potential forincompatibility of coating layers due to, for example, coefficient ofthermal expansion mismatch between layers, etc.

Accordingly, a long-felt need has existed for compositions which may beused to provide enhanced protection against, for example, oxidation,hydrolysis, etc., when applied to oxidizable substrates, without therequirement for expensive and/or complex processes.

SUMMARY OF THE INVENTION

The present invention relates to novel processes for formingcompositions which may be utilized, among other things, as protectivecoatings on surfaces. Particularly, the present invention is directed tocompositions derived from polymers containing metal-nitrogen bonds,which compositions exhibit, among other things, desirable oxidationresistance, corrosion resistance and hydrolytic stability when exposedto adverse environments, whether at ambient or elevated temperatures,and which may be useful as, for example, protective coatings onsurfaces.

In a preferred embodiment of the present invention, a composition isfirst produced by admixing at least one boron source comprising anextended network of boron-boron bonds and at least one polymercomprising silicon-nitrogen bonds. The composition is then subjected tothe introduction of energy, such as by heating, etc., in the presence ofat least one oxidant. As used herein, the term "oxidant" means one ormore suitable electron acceptors or electron sharers and may be anelement, combination of elements, a compound, or combination ofcompounds including reducible compounds, and may be vapor, liquid orsolid, or a combination thereof, at the process conditions. Withoutwishing to be bound by any particular theory or explanation, when the atleast one oxidant comprises oxygen, it is believed that the oxygen fromthe oxidant can be chemically incorporated into the composition. In apreferred embodiment of the present invention, whereby the compositionis used as a protective coating on at least a portion of at least onesubstrate, the at least one coated substrate is subjected to heating,either during or subsequent to said coating procedure, in the presenceof at least one oxidant, such heating occurring at a sufficienttemperature and for a sufficient time to permit at least some chemicalreaction of the composition with at least the at least one substrate toform a chemically bonded protective coating thereon. The chemicalreaction which occurs may result in a bonding zone which may be presentonly on a molecular level (i.e., reaction of single atoms at thesubstrate-coating interface) or may extend at least partially throughsaid coating. In a particularly preferred embodiment of the presentinvention, the at least one coated substrate is heated in air at atemperature of at least about 250° C., and in a particularly preferredembodiment at least about 400° C., for a time sufficient to form saidprotective coating. In certain embodiments, mechanical bonding of thecoating with at least a portion of the substrate may occur.Alternatively, some combination of chemical reaction and mechanicalbonding may occur. The resulting coating comprises a highly adherentcomposition which exhibits, among other things, excellent resistance tooxidation and to corrosion and hydrolytic stability when exposed to suchenvironments.

In an alternative preferred embodiment of the present invention,coatings comprising multiple layers of protective compositions may beformed to achieve a desired result. Depending on the desiredapplication, the composition of one layer within said coating may besubstantially similar to or substantially different from the compositionof at least one adjacent layer. Additionally, the thickness of saidcoating layers may be tailored to achieve a desired result. Individualcoating layer thicknesses may range significantly depending on thespecific desired application; however, typical coating layer thicknessesrange from about 0.001 mm to about 0.5 mm, and most preferably in therange of about 0.025 mm to about 0.1 mm. Moreover, depending on thedesired application, the thickness of a given coating layer may vary inone region relative to another region.

In a further preferred embodiment of the present invention, thecomposition may comprise at least one filler material in at least aportion thereof. The method of the present invention is applicable to awide variety of filler materials, and the choice of filler materialswill depend on such factors as the starting materials used to form thecomposition, the properties sought for the final composition, theprocessing conditions, substrate compositions, etc.

The compositions of the present invention may be used in any number ofapplications including, but not limited to, coherent bodies, such asthree-dimensional articles, sheets, or the like, protective coatings,adhesives, sealants, binders, as well as any other applicationcontemplated by one skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention relates to novel processes for formingcompositions which may be utilized, among other things, as protectivecoatings on surfaces. Particularly, the present invention is directed tocompositions derived from polymers containing metal-nitrogen bonds,which compositions exhibit, among other things, desirable oxidationresistance, corrosion resistance and hydrolytic stability when exposedto adverse environments, whether at ambient or elevated temperatures,and which may be useful as, for example, protective coatings onsurfaces.

In a preferred embodiment of the present invention, a composition isfirst produced by admixing at least one boron source comprising anextended network of boron-boron bonds, and at least one polymercomprising silicon-nitrogen bonds. The composition is then subjected tothe introduction of energy, such as by heating, etc., in the presence ofat least one oxidant. As used herein, the term "oxidant" means one ormore electron acceptors or electron sharers and may be an element,combination of elements, a compound, or combination of compoundsincluding reducible compounds, and may be in vapor, liquid or solidform, or some combination thereof, at the process conditions. Withoutwishing to be bound by any particular theory or explanation, when the atleast one oxidant comprises oxygen, it is believed that the oxygen fromthe oxidant can be chemically incorporated into the composition. In apreferred embodiment of the present invention, whereby the compositionis used as a protective coating on at least a portion of at least onesubstrate, and the at least one coated substrate is subjected toheating, either during or subsequent to said coating procedure, in thepresence of the at least one oxidant, such heating occurring at asufficient temperature and for a sufficient time to permit at least somechemical reaction of the composition with the at least one substrate toform a chemically bonded protective coating thereon. The chemicalreaction which occurs may result in a bonding zone which may be presentonly on a molecular level (i.e., reaction of single atoms at thesubstrate-coating interface) or may extend at least partially throughsaid coating. In a particularly preferred embodiment of the presentinvention, the at least one coated substrate is heated in air at atemperature of at least about 250° C., and in a particularly preferredembodiment at least about 400° C. in air, for a time sufficient to formsaid protective coating. In certain embodiments, mechanical bonding ofthe coating with at least a portion of the substrate may occur.Alternatively, some combination of chemical reaction and mechanicalbonding may occur. The resulting coating comprises a highly adherentlayer which exhibits, among other things, excellent resistance tooxidation and to corrosion and hydrolytic stability when exposed to suchenvironments.

In an alternative preferred embodiment of the present invention,coatings comprising multiple layers of protective compositions may beformed to achieve a desired result. Depending on the desiredapplication, the composition of one layer within said coating may besubstantially similar to or substantially different from the compositionof at least one adjacent layer. Additionally, the thickness of saidcoating layers may be tailored to achieve a desired result. Individualcoating layer thicknesses may range significantly, depending on thespecific application; however, typical coating layer thicknesses rangefrom about 0.001 mm to about 0.5 mm, and most preferably in the range ofabout 0.025 mm to about 0.1 mm. Moreover, depending on the desiredapplication, the thickness of a given coating layer may vary in oneregion relative to another region.

Polymers suitable for the practice of the instant invention includepolymers which comprise at least some repeating structural units whichcomprise at least some metal-nitrogen bonds. Suitable metal-nitrogenpolymers which may be used in the method of the present inventioninclude, but are not limited to, polymers comprising, for example,silicon-nitrogen bonds, aluminum-nitrogen bonds, boron-nitrogen bonds,and combinations thereof. Particularly preferred polymers comprisingmetal-nitrogen bonds for use in the present invention comprise polymerscomprising at least some silicon-nitrogen bonds. For the purposes of thepresent invention, the term `polymer` includes compounds having at leastabout 10 repeating structural units. Particularly preferred polymers forthe method of the present invention comprise those polymers wherein atleast about 50 percent of said repeating structural units comprisesilicon-nitrogen bonds. The polymers preferably have a low vaporpressure at temperatures below about 400° C. to enable retention andreaction between at least the boron source and the underlying substratein the presence of the at least one oxidant. Moreover, the preferredpolymers of the present invention comprise fusible polymers in order tofacilitate application as a coating. The polymers should preferablycomprise polymers which crosslink upon the introduction of energy, suchas, for example, by thermosetting upon heating. By the term `thermoset`is meant the development of a shape-stabilized structure through theapplication of heat. Such a structure is typically achieved through thedevelopment of an extended network of chemical crosslinks betweenpolymer chains. Polysilazanes, polyureasilazanes, andpolythioureasilazanes are representative of such thermosettablepolymers. Most preferred for the method of the present invention arepolysilazane and polyureasilazane compositions which further comprisesites of organounsaturation, such as alkenyl or alkynyl groups which maybe crosslinked to rigid, solid materials through thermosetting or othercomparable means. Such coating compositions may optionally comprise freeradical generators to assist in crosslinking.

Preferred polymers for use in the method of the present inventioncomprise liquid polymers which can be crosslinked using an energy inputsuch as heat or UV irradiation. Particularly preferred polymers compriseliquid polysilazanes and polyureasilazanes which can be crosslinked tosolid materials through the application of an energy input, such asthose polymer compositions described in U.S. Pat. Nos. 4,929,704,5,001,090 and 5,021,533, all in the name of Joanne M. Schwark, whichpatents are specifically incorporated by reference herein in theirentirety. Other particularly preferred polymers include those polymercompositions described in U.S. Pat. No. 4,722,988. in the names of Porteet al., and U.S. Pat. No. 4,689,252, in the names of LeBrun et al.,which patents are also specifically incorporated by reference herein intheir entirety.

Solid polymers may be used in the method of the present invention byadding a solvent to the composition or by providing as a melt. Preferredsolvents typically comprise organic solvents which are substantiallynon-reactive with the polymer. While the choice of solvents depend onthe exact chemical composition of the polymers utilized, and may bereadily determined by one skilled in the art, useful solvents mayinclude, for example, hydrocarbon solvents such as hexane, heptane,toluene and xylene, ethers such as diethyl ether and tetrahydrofuran, orhalogenated solvents such as chlorobenzene or tetrachloroethane.Solvents which contain oxygen may under certain conditions act as anoxidant, either alone or in combination with at least one additionaloxidant, to assist in the reaction to form the protective composition.Further, solvents which contain oxygen may be reactive or non-reactivewith the polymer at room temperature. Thus, alcohols, such as methanol,ethanol and isopropanol which react desirably with polysilazanes at roomtemperature may be used. Depending on the system chosen, in certaininstances, rheology and coating capability of the composition may beaffected by the solvent. In addition, oxygenated solvents which do notreact or react only minimally with the polysilazane at room temperature,but which may react with the polysilazane at elevated temperatures, maybe used. Such oxygenated solvents include, but are not limited to,ketones such as acetone or methyl ethyl ketone and esters such asdimethyl adipate.

Boron sources particularly suitable for use in the method of the presentinvention exhibit an extended network of boron-boron bonds. Thus, thenumber of sequential boron-to-boron bonds in the boron source shouldexceed one to achieve such extended network. Representative structuresof boron-boron bond networks in the practice of the present inventioninclude chain, sheet, and three dimensional cage structures. Sheet andcage structures are preferred embodiments for use in the presentinvention, while cage structures are particularly preferred embodiments.Examples of cage structures include, but are not limited to, crystallinepolymorphs of boron and metal borides comprising the icosahedral boronstructure including, for example, alpha-rhombohedral B₁₂,beta-rhombohedral B₁₀₅, and tetragonal B₅₀ structures. While not wishingto be bound by any particular theory or explanation, it is believed thatan important feature in the use of the cage structures is the oxidativebehavior of such structures. These boron-rich structures, particularlyin the case of the icosahedral moieties, have been observed to oxidizewith full retention of the icosahedral boron cage structures to formcompounds which comprise suboxides of boron, rather than stoichiometricB₂ O₃. Suboxide compounds such as, for example, B₁₂.OBO, are, typically,highly refractory materials with excellent properties, includingexcellent hardness, oxidation resistance, corrosion resistance, etc.Thus, the utilization of boron sources comprising extended boron-boronbond networks in the method of the present invention results incompositions which provide desirable protective properties at hightemperatures to the substrates onto which they may be coated. Incontrast, the predominant oxidation product of amorphous boron or otherboron compounds which do not exhibit extended boron-boron bonding, suchas B(OH)₃, organic or inorganic borates, boron nitride, borazines orboroxines, is amorphous or vitreous boron oxide, B₂ O₃, which melts atabout 450° C. Such boron sources are often capable of being partiallyoxidized to form compounds which may be useful at intermediatetemperatures (e.g., temperatures at which complete conversion to B₂ O₃does not occur).

Boron sources suitable for the method of the present invention maycomprise either fusible boron sources or infusible boron sources.Examples of fusible boron sources include, but are not limited to,boranes, carboranes and boron-containing polymers which are eitherliquids or meltable or solvent-soluble solids. Examples of infusibleboron sources include, but are not limited to, particulate or plateyinorganic boron compounds such as metal borides, etc. The boron sourcespreferably have low vapor pressures at temperatures at or below about400° C. to enable retention and reaction with the silicon-nitrogenpolymer, the at least one oxidant and the underlying substrate. Examplesof boron sources suitable for use in this invention include, but are notlimited to, boron carbide, silicon boride, including silicon hexaborideand silicon tetraboride, borides of transition metals, the lanthanideseries, alkaline earth or rare earth metals, and other such borides aslisted in Table I, below, derived from Table 24.3 of StructuralInorganic Chemistry (1984, Wells, A. F. Oxford Science Publications, 5thed. p. 1053).

                  TABLE I                                                         ______________________________________                                        Typical Crystal Structures of Some Metal Borides                              NATURE OF                                                                     B COMPLEX  STRUCTURE   EXAMPLES                                               ______________________________________                                        Single chains                                                                            CrB         NbB, TaB, VB                                                      FeB         CoB, MnB, TiB, HfB                                                MoB         WB                                                                Ni.sub.4 B.sub.3                                                              Ru.sub.11 B.sub.8                                                  Double chains                                                                            Ta.sub.3 B.sub.4                                                                          Cr.sub.3 B.sub.4, Mn.sub.3 B.sub.4, Nb.sub.3                                  B.sub.4                                                Layers     AlB.sub.2   M = Ti, Zr, Nb, Ta, V,                                                        Cr, Mo, U                                                         ε-Mo-B                                                                            ε-W-B phase                                    3D frameworks                                                                            CrB.sub.4                                                                     UB.sub.4    M = Y, La, Th, and                                                            the lanthanide series                                             CaB.sub.6   M = Sr, Ba, Y, La, Th, U,                                                     and the lanthanide series                                         UB.sub.12   M = Y, Zr, and the                                                            lanthanide series                                      ______________________________________                                    

In a preferred embodiment for preparing the compositions of the presentinvention, from about 5 percent to about 95 percent, by weight of thetotal mixture of the boron source may be used. In a particularlypreferred embodiment, the ratio of the boron source is about 10 percentto about 80 percent by weight of the total mixture. The polymercomprising silicon-nitrogen bonds may preferably be included in a ratioof from about 5 percent to about 95 percent by weight of the totalmixture. In a particularly preferred embodiment, from about 20 percentto about 90 percent by weight of the total mixture of silicon-nitrogenpolymer may be used. Moreover, the amount of silicon-nitrogen polymerutilized may be tailored to obtain a desired consistency of thecomposition.

In a further preferred embodiment of the present invention, thecomposition may comprise at least one filler material in at least aportion thereof. The method of the present invention is applicable to awide variety of filler materials, and the choice of filler materialswill depend on such factors as the starting materials used to form thecomposition, the properties sought for the final composition, theprocessing conditions, substrate compositions, etc. Fillers suitable foruse in the present invention include, but are not limited to, suchmaterials as ceramics, glasses, metals, organic and inorganic monomers,oligomers and polymers, hybrid polymers, and mixtures thereof, whichfillers may or may not react with the composition. Representativeceramics include oxides, carbides, borides, nitrides and silicides, suchas, for example, magnesium oxides, aluminum oxides, chromium oxides,titanium oxides, zirconium oxides, titanium nitrides, boron nitrides,silicon nitrides, silicon carbides, titanium carbides, zirconiumcarbides, chromium carbides, titanium borides, zirconium borides,titanium silicides and molybdenum silicides. Representative metalfillers suitable for the practice of the present invention include, butare not limited to, nickel, aluminum, zinc, chromium, steel, stainlesssteel, and metal alloys such as nickel-based alloys including, but notlimited to, Incoloy, INCONEL® and HASTELLOY®. Organic and inorganicfillers suitable for use in the present invention may include, but arenot limited to such fillers as carbon, oligomers and polymers of organiccompounds such as urethanes, epoxies, imides, amides, esters,carbamates, carbonates, phenolics, silanes, siloxanes, silazanes,phosphazenes, alazanes, and borazines. In a particularly preferredembodiment, polymeric fillers may comprise fluoropolymers such aspolytetrafluoroethylene, polyvinylidine fluoride, fluorinatedethylene-propylene and perfluoroalkoxy polymer. Representative hybridpolymers which may be used as fillers in the method of the presentinvention include those hybrid polymers comprising at least one organicelectrophile and at least one metal-containing polymer, as described inU.S. patent application Ser. No. 08/148,044, filed on Nov. 5, 1994, inthe names of Becker et al. and U.S. patent application Ser. No.08/223,294, filed Apr. 5, 1994, also in the names of Becker et al., thesubject matter of each of which is incorporated herein by reference inits entirety.

Fillers may be provided in a wide variety of forms and sizes, such aspowders, particulates, flakes, platelets, spheres and microspheres (bothhollow and solid), whiskers, bubbles, continuous fibers, yarns or tows,chopped fibers, whiskers, fiber mats, three-dimensional wovenstructures, or the like, and combinations thereof. Moreover, fillers maypreferably be provided in an amount of from about 1 percent to about 60percent by volume of the total composition, and most preferably betweenabout 10 percent and about 50 percent by volume of the total mixture.

The use of filler materials of varying geometries, volume percentagesand compositions in the method of the present invention, along with thecapability to tailor the composition itself, permits the capability totailor the properties of compositions produced by the method of thepresent invention to achieve a virtually unlimited number of possiblecombinations of properties. For example, in a preferred embodiment ofthe present invention for forming a protective coating on a substrate,the composition may be tailored to include at least one filler which ischosen such that it provides a coating having a linear thermal expansioncoefficient close to that of the substrate being coated. Coefficient ofthermal expansion matching provides for the retention of coatingintegrity during thermal cycling and prevents coating spalling due tostresses imposed during cool-down. Moreover, in another preferredembodiment of the present invention, when at least one filler comprisingflakes such as, for example, stainless steel leafing grade flakes isincorporated into the composition to be coated onto a substrate as aprotective layer, a seal of overlapping stainless steel flakes isachieved, providing, among other things, enhanced oxidation andcorrosion resistance. Moreover, although not wishing to be bound by anyparticular theory or explanation, it is believed that the overlappingnature of the filler material in the form of flakes provides additionalprotection upon exposure to an undesirable environment by providing, forexample, a more tortuous path through which any undesirable environmentwhich is able to penetrate the coating composition must traverse beforereaching the substrate. In an alternative preferred embodiment, theaddition of ceramic filler compositions such as, for example, siliconcarbide filler to the composition may increase, among other things, theabrasion resistance of the substrate surface.

The sizes and geometries of the components utilized to form thecompositions of the present invention may vary significantly dependingon the desired final composition. In a particularly preferred embodimentof the present invention, particle size of any particulates which may bepresent in the mixture (e.g., boron source, filler, etc. ) should bebetween about 0.1 μm and 100 μm, and most preferably between about 0.1μm and 10 μm. Moreover, as discussed earlier herein, the compositions ofthe present invention may be either homogeneous or heterogeneousdepending on the desired final properties, etc., of the compositions.When a homogeneous composition is preferred for a given composition, thecomposition may be processed in a manner sufficient to ensure such ahomogeneous mixture using any technique, such as by milling, etc., whichwould be readily apparent to one skilled in the art.

The compositions of the present invention may be used in any number ofapplications including, but not limited to, coherent bodies, such asthree-dimensional articles, sheets, or the like, protective coatings,adhesives, sealants, binders, as well as any other applicationcontemplated by one skilled in the art.

In a particularly preferred embodiment of the present invention, thecompositions of the present invention may be utilized as protectivecoatings on a variety of substrate materials. Substrates suitable forcoating by the method of the present invention include, but are notlimited to, metals, metal alloys, metal matrix composites,intermetallics, minerals, carbon, organic and inorganic oligomers andpolymers, hybrid polymers, glasses, ceramics, ceramic composites, andthe like. The geometries of the substrates which may be coated utilizingthe compositions of the present invention may include, but are notlimited to, coherent bodies, powders, particulates, continuous fibers,yarns or tows, chopped fibers, whiskers, fiber mats, three-dimensionalwoven structures, or the like. In a particularly preferred embodiment ofthe present invention, the substrates may comprise at least partiallyoxidizable substrates, and most particularly substrates which canincorporate oxygen to form stable, non-volatile oxides. Representativeof the oxidizable substrates which are particularly useful in thepractice of the present invention are ferrous metals such as cast iron,mild steel, and stainless steel, as well as non-ferrous metals such asaluminum and aluminum-silicon alloys. Also suitable are compounds whichcan oxidize to form metal-oxide comprising compositions including, butnot limited to metal borides, metal nitrides, metal carbides, metalsulfides, and metal phosphides, including but not limited to substratessuch as silicon carbide, silicon nitride, titanium nitride, aluminumnitride, and titanium diboride.

The compositions of the present invention are suitable for coating ontosurfaces of substrates without the requirement for any special substratesurface preparation. In a particularly preferred embodiment, it wasunexpectedly observed that for some oxidizable metals, such as ferrousmetals, treatment of the substrate surface, such as cleaning oroxidation of the surface, is typically not required in the process ofthe present invention. Without wishing to be bound by any particulartheory or explanation, it is believed that such surface treatment is notrequired due to the presence of oxygen-containing groups, such ashydroxyl groups or the like, on at least a portion of the nativesubstrate surfaces which enhance formation of the coatings; this featureof the present invention is particularly useful when coating surfacescontaining at least some rust, or the like. This novel feature isparticularly unexpected in view of the teachings of prior art coatingtechniques where surface preparation can be crucial to coating adhesion.However, if desired in a given embodiment, the substrate surfaces may bemodified to enhance coating by any of a number of techniques. Physicalmeans, such as sandblasting and grit blasting, or chemical oxidation,which can be accomplished by etching the surface with an inorganic acidsuch as phosphoric acid or chromic acid and/or anodizing, are suitablemethods for modifying the substrate surfaces, the selection of which maydepend on the particular substrate. Alternatively, air oxidation may beused to prepare the substrate for coating.

Applying the protective composition to at least a portion of at leastone substrate surface may be accomplished by any number of conventionalmethods. Examples of preferred techniques include, but are not limitedto, brush-coating, spray-coating, dip-coating, and the like. Forexample, in a preferred embodiment, coating compositions which comprisea meltable polymer may be applied by dipping the substrate into a melt.Moreover, fluid compositions comprising either a solvent or a liquidmetal-nitrogen polymer may be applied, for example, by brush, dip, orspray coating techniques. In another embodiment of the presentinvention, when heat-curable liquid coating compositions are utilized,the coating may be applied by hot-spraying, i.e., spraying the coatingonto a substrate which has been heated to a temperature above ambienttemperature. In a particularly preferred embodiment, the substrates maybe heated, preferably to at least about 100° C., prior to coating. Uponcontact with the heated substrate, the coating composition cures on thesurface, obviating additional processing steps and creating a simple,economical method of forming multiple coating layers in a continuousapplication process. Multiple coatings may be applied where it isdesirable to use coatings to provide separate protection-specificproperties. For example, in a particularly preferred embodiment.Multiple coatings on mild steel may be applied whereby zirconium oxideis added as a filler to the protective composition of the top layer toprovide resistance to, for example, acidic environments, and stainlesssteel flake may be added to the coating composition of the bottom layerto provide resistance to, for example, abrasion.

In a particularly preferred embodiment of the present invention whereinthe compositions are utilized as protective coatings, such coatings maybe formed by heating the mixture comprising the at least one polymercomprising silicon-nitrogen bonds and the at least one boron sourceunder appropriate processing conditions. In a particularly preferredembodiment of the present invention, the coating may be formed byheating the mixture of the at least one polymer containingsilicon-nitrogen bonds and the at least one boron source in the presenceof at least one oxidant. Temperatures greater than 250° C. arepreferred, with temperatures greater than 400° C. being particularlypreferred, depending on the substrates utilized. For particularlydurable substrates, such as refractory metals, ceramics and stainlesssteel, temperatures as high as 1000° C. or higher may be used.

The nature of the protective coating depends on the temperature to whichthe coating is heated. In low temperature ranges, for example less thanabout 500° C., the nature of the coating comprises predominantly that ofa highly crosslinked polymer. At higher temperatures, for example about500° C. to 1000° C., the coating takes on predominantly amorphouscharacteristics. At the highest temperature ranges, for exampletemperatures greater than about 1000° C., predominantly amorphous orcrystalline ceramic characteristics may be obtained.

Oxidants which are preferred in the method of the present inventioninclude, but are not limited to, oxygen, air, mixtures of oxygen withinert or other oxidizing gases, and the like. Particularly preferredoxidants in the present invention comprise air, oxygen and otheroxidizing atmospheres which further comprise water vapor, such as watervapor-saturated air. Without wishing to be bound by any particulartheory or explanation, it is believed that the water vapor may aid inthe oxidation of the coating composition by providing partial hydrolysisof the silicon-nitrogen bonds in the at least one polymer. In addition,when at least one filler is incorporated into the compositions, thewater vapor may also at least partially oxidize the at least one filler.

Various demonstrations of the present invention are included in theExamples immediately following. However, these Examples should beconsidered as illustrative only and not be construed as limiting thescope of the invention as defined in the appended claims. Additionally,a number of Comparative Examples are provided.

EXAMPLE 1

The present Example demonstrates, among other things, the preparation ofa polysilazane: [(CH₃ SiHNH)₀.8 (CH₂ ═CHSiCH₃ NH)₀.2 ]_(x) which can beused to form a coating on a substrate. An about 5 liter, three-neckedflask was equipped with an overhead mechanical stirrer, a dryice/acetone condenser (about -78° C.), an ammonia or nitrogen inlet tubeand a thermometer to form an apparatus. The interior of the apparatuswas sparged with nitrogen. The apparatus was then charged with hexane(about 1760 milliliters (ml), dried over Type 4A molecular sieves),methyldichlorosilane (about 209 ml, about 230.9 grams, about 2.0 mol)and vinylmethyldichlorosilane (about 64 ml, about 69.6 grams, about 0.5mol). Ammonia was added to the apparatus at a rate of about 3.5 litersper minute (about 9.37 mol) for about one hour. During the ammoniaaddition, the temperature of the contents of the apparatus rose fromabout 25° C. to about 69° C. After about one hour, the ammonia flow wasstopped and the reaction mixture was allowed to cool to about roomtemperature. The reaction mixture was filtered on a glass-fritted funnelto remove any precipitated ammonium chloride. The hexane was removedfrom the filtrate under reduced pressure of about 2 millimeter (mm)mercury (Hg) (0.079 inch Hg) to give a polysilazane [(CH₃ SiHNH)₀.8 (CH₂═CHSiCH₃ NH)₀.2 ]_(x), as a clear oil (about 150.8 gram, about 2.34 mol,about 94 percent yield) having a viscosity of about 43 cemipoise atabout 25° C., and a molecular weight of about 560 grams/mol.

EXAMPLE 2

The present Example demonstrates, among other things, the preparation ofa polyureasilazane which can be used to form a protective coating on asubstrate. A methylvinylpolyureasilazane was prepared substantially bythe method of U.S. Pat. No. 4,929,704. Specifically, an about 100milliliter (ml) one-necked flask equipped with a stir bar and a septumwas sparged with nitrogen and charged with about 10.0 grams of thepolysilazane, [(CH₃ SiHNH)₀.8 (CH₂ ═CHSiCH₃ NH)₀.2 ]_(x), preparedsubstantially as described in Example 1 and about 0.2 weight percentphenyl isocyanate. The flask was placed in an oil bath on a stirrer/hotplate and the septum was replaced with a water condenser capped with aseptum. A nitrogen inlet and oil bubbler outlet were placed in theseptum. The reaction mixture was then heated to about 110° C. under anitrogen atmosphere for about 17 hours. Evolution of hydrogen gas wasobserved. After cooling to about room temperature, the viscosity of theliquid polyureasilazane product measured about 300 centipoise.

EXAMPLE 3

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and zinc. A stainless steel substrate (Series 304) measuringabout 8 cm by 8 cm by 0.1 cm was prepared for coating by sandblastingthe surface. The substrate was then coated by brush coating a mixtureonto the substrate. The coating mixture comprised about 50 percent byweight of zinc dust (˜66 μm, L2-1144, #136, leafing grade, Canbro,Quebec, Canada), about 25 percent by weight boron carbide (B₄ C, 5 μm,ESK-Engineered Ceramics, New Canaan, Conn.), and 25 percent by weightpolyureasilazane prepared substantially according to the method ofExample 2 (and with the addition of about 0.5 percent dicumyl peroxideby weight of the polyureasilazane). The coated substrate was heated toabout 150° C. in air for about 30 minutes, then heated to about 500° C.in air for about 3 hours. The coated substrate remained unaffected bycorrosion when exposed to a saturated aqueous sodium chloride solutionfor about 24 hours at ambient temperature.

EXAMPLE 4

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and silicon carbide. Example 3 was substantially repeated exceptthat the coating mixture was comprised of about 50 percent by weight ofpolyureasilazane prepared substantially according to the method ofExample 2 (and with the addition of about 0.5 percent dicumyl peroxideby weight of the polyureasilazane), about 25 percent by weight of boroncarbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.), andabout 25 percent by weight silicon carbide (E-110, 5 μm Norton,Liliesand, Norway). The coated substrate was heated to about 150° C. inair, then heated to about 600° C. in air. The coated substrate remainedunaffected by corrosion when exposed to a saturated aqueous sodiumchloride solution at ambient temperature for about 24 hours.

EXAMPLE 5

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and zirconium oxide. Example 3 was substantially repeated exceptthat the coating mixture was comprised of about 25 percent by weight ofpolyureasilazane prepared substantially according to the method ofExample 2 (and with the addition of 0.5 percent dicumyl peroxide byweight of the polyureasilazane), about 15 percent by weight of boroncarbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.), andabout 60 percent by weight zirconium oxide (0.5 μm, SC-15 grade,Magnesium Elektron, Inc., Flemington, N.J.). The coated substrate washeated to about 150° C. in air for about 30 minutes, then heated toabout 800° C. in air for about 3 hours. The coated substrate remainedunaffected by corrosion when exposed to a saturated aqueous sodiumchloride solution at ambient temperature for about 1 week.

EXAMPLE 6

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and silicon carbide. Example 3 was substantially repeated exceptthat the coating mixture comprised about 30 percent by weight ofpolyureasilazane prepared substantially according to the method ofExample 2 (and with the addition of about 0.5 percent dicumyl peroxideby weight of the polyureasilazane and about 5 percent toluene), about 10percent by weight of boron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics,New Canaan, Conn.), and about 60 percent by weight silicon carbide(E-110, 5 μm, Norton, Liliesand, Norway). The coated substrate washeated to about 150° C. in air for about 30 minutes, and then heated toabout 600° C. in air for about 3 hours. The coated substratedemonstrated resistance to corrosion when exposed to concentratedsulfuric acid at ambient temperature for about 6 hours.

EXAMPLE 7

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and silicon carbide. Example 6 was essentially repeated exceptthat the formed coating was exposed to saturated aqueous sodium chloridesolution at ambient temperature for about 1 day and demonstratedresistance to corrosion.

EXAMPLE 8

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and silicon carbide. Example 3 was substantially repeated exceptthat the coating mixture was comprised of about 40 percent by weight ofpolyureasilazane prepared substantially according to the method ofExample 2 (and with the addition of about 0.5 percent dicumyl peroxideby weight of the polyureasilazane and about 5 percent toluene), about 20percent by weight of boron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics,New Canaan, Conn.), and about 40 percent by weight silicon carbide(E-110, 5 μm, Norton, Liliesand, Norway). The coated substrate washeated to about 150° C. in air for about 30 minutes, and heated to about600° C. in air for about 3 hours. The coated substrate demonstratedresistance to corrosion when exposed to concentrated sulfuric acid atambient temperature for about 2 days.

EXAMPLE 9

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and silicon carbide. Example 6 was essentially repeated exceptthat the formed coating was exposed to concentrated hydrochloric acid atambient temperature for about 2 days and demonstrated resistance tocorrosion.

EXAMPLE 10

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and zirconium oxide. A stainless steel substrate (series 310)measuring about 8 cm by 8 cm by 0.1 cm was prepared for coating bysandblasting the surface. The substrate was then coated by spraying amixture comprising about 25 percent by weight polyureasilazane preparedsubstantially according to the method of Example 2, about 60 percent byweight zirconium oxide (0.5 μm, SC-15 grade, Magnesium Elektron, Inc.,Flemington, N.J.), and about 15 percent by weight boron carbide (5 μm B₄C, ESK-Engineered Ceramics, New Canaan, Conn.)) which had been milledfor about 12 hours. A coating of about 50 to 60 μm thickness was appliedto the substrate surface using a Paasche Model H airbrush with a dualaction external mix and a #3 needle with a #5 cone. The coated substratewas heated in air at about 150° C. for about 15 minutes, and then washeated to about 200° C. in air. The temperature was increased to about500° C. at a rate of about 100° C. per hour and held at about 500° C.for about 1 hour, after which time the temperature was increased toabout 800° C. at about 200° C. per hour and held at about 800° C. forabout 1 hour. The coated substrate was then cooled to room temperature.

EXAMPLE 11

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, boroncarbide and stainless steel filler. Example 10 was essentially repeated,except that the liquid mixture was prepared by combining about 175 gramsof polyureasilazane prepared substantially according to the method ofExample 2, about 32 grams of boron carbide (5 μm, B₄ C fromESK-Engineered Ceramics, New Canaan, Conn.), and about 60 grams 316stainless steel flakes (15-40 μm, 316×55 leafing grade, U.S. BronzePowders, Inc., Flemington, N.J.), and milling for about 1 hour. Thecoating was tested by heating to about 850° C. for about 24 hours in anoxidizing atmosphere, and was observed to remain adherent and unaffectedby oxidation.

EXAMPLE 12

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane,silicon boride and silicon carbide. Example 10 was substantiallyrepeated except that the coating mixture comprised about 50 grams ofpolyureasilazane prepared substantially according to the method ofExample 2, about 10 grams of silicon tetraboride (˜66 μm, Cerac/PureAdvanced Specialty Inorganics, Milwaukee, Wis.), and about 10 grams ofsilicon carbide (E-110, 5 μm, Norton, Liliesand, Norway). The coatedsubstrate was heated to about 200° C. for about 30 minutes in air, thenheated to about 500° C. for about 1 hour in air, and then heated toabout 800° C. for about 30 minutes in air. After quenching in air atroom temperature, the coating was immersed in water for about 72 hours,and was subjected to a scratch test to determine hydrolytic stabilityand adherence, respectively. It was observed that the coating remainedadherent and unaffected by oxidation and hydrolysis.

EXAMPLE 13

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, nickeland boron carbide. Example 10 was essentially repeated except that thecoating mixture comprised about 80 grams of polyureasilazane preparedsubstantially according to Example 2, about 40 grams of nickel leafpowder (15-40 μm, fine leafing grade flake, Novamet, Wycoff, N.J.), andabout 12 grams of boron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics,New Canaan, Conn.). The coating remained adherent and unaffected byoxidation when heated to about 850° C. for about 24 hours.

EXAMPLE 14

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane,silicon boride and silicon carbide. Example 10 was substantiallyrepeated, except that the coating mixture comprised about 50 grams ofpolyureasilazane prepared substantially according to Example 2, about 10grams of silicon hexaboride (˜66 μm, Cerac/Pure Advanced SpecialtyInorganics, Milwaukee, Wis.), and about 10 grams of silicon carbide(E-110, 5 μm, Norton, Liliesand, Norway). The coated substrate washeated to about 200° C. for about 30 minutes in air, then heated toabout 500° C. in air for about 1 hour, and then heated to about 800° C.in air for about 30 minutes. The coating remained adherent andunaffected by hydrolysis after it was immersed in water for about 72hours and then subjected to a scratch test.

EXAMPLE 15

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane,lanthanum boride and silicon carbide. Example 10 was substantiallyrepeated except that the coating mixture was comprised of about 75 gramspolyureasilazane prepared substantially according to Example 2, about 15grams of lanthanum hexaboride (40 μm, Cerac/Pure Advanced SpecialtyInorganics, Milwaukee, Wis.), and about 15 grams of silicon carbide(E-110, 5 μm, Norton, Liliesand, Norway). The coated substrate washeated to about 200° C. in air for about 30 minutes, then heated toabout 500° C. in air for about 1 hour and then heated to about 800° C.for about 30 minutes. The coating remained adherent and unaffected byhydrolysis after it was immersed in water for about 72 hours and thensubjected to a scratch test.

EXAMPLE 16

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane,aluminum boride and silicon carbide. Example 10 was substantiallyrepeated except that the mixture was prepared by combining about 50grams of polyureasilazane prepared substantially according to the methodof Example 2, about 10 grams of aluminum boride (AlB₁₂, 3-8 μm m, AlfaProducts, Danvers, Mass.), and about 10 grams of silicon carbide (E-110,5 μm, Norton, Liliesand, Norway). The coating was brushed onto thesurface and the coated substrate was heated to about 500° C. for about 1hour in air, and then was heated to about 800° C. for about 30 minutesin air. The coated substrate was unaffected by water after about 72hours of immersion.

EXAMPLE 17

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane, bariumboride and silicon carbide. Example 10 was substantially repeated exceptthat the mixture was prepared by combining about 50 grams ofpolyureasilazane prepared substantially according to the method ofExample 2, about 10 grams of barium hexaboride (40 μm, BaB₆, AlfaProducts, Danvers, Mass.), and about 10 grams of silicon carbide (E-110,5 μm, Norton, Liliesand, Norway). The coating was brushed onto thesurface and the coated substrate was heated to about 500° C. for about 1hour in air, and then was heated to about 800° C. for about 30 minutesin air. The coated substrate was unaffected by water after about 72hours of immersion.

EXAMPLE 18

The following example demonstrates, among other things, the formation ofa coating composition from a mixture comprising silicon boride andpolyureasilazane. A cylindrical graphite disc measuring about 5 cm indiameter by about 0.6 cm in height and weighing about 22.01 grams wasbrush coated with a mixture comprising about 70 grams ofpolyureasilazane made substantially according to the method of Example 2(and with the addition of about 0.5 percent dicumyl peroxide by weightof the polyureasilazane), and about 30 grams of silicon hexaboride (˜66μm, Cerac/Pure Advanced Specialty Inorganics, Milwaukee, Wis.). Thecoated disc was heated at about 150° C. for about 30 minutes in an airatmosphere to cure the polyureasilazane. After curing, the coatedsubstrate weighed about 22.52 grams. The coated substrate was thenheated in an air atmosphere from room temperature to about 450° C. inabout 2 hours and held at about 450° C. for about 3 hours. The coatedsubstrate obtained was brown in color and weighed 22.74 grams, thusexhibiting a total weight gain of the coating of about 43 percent. Thesample was then immersed in water to soak for about 16 hours at ambienttemperature, after which time the water remained clear and colorlesswith no noticeable particulates present. The disc was removed from thewater, dried by paper towel and then further dried by heating at about150° C. for about 2 hours. The disc was then reweighed. The dried,coated disc weighed about 22.72 grams.

The coated disc was then placed in a stainless steel holder whichallowed over 99 percent of the disc surface to be directly exposed tohot ambient, convecting air. The holder and the specimen were placed ina box furnace which had been heated to about 650° C. Approximately every3 to 4 hours the holder and specimen were removed from the furnace andquenched in ambient air, the specimen was weighed and remounted in theholder, and then the holder and specimen were replaced in the heatedfurnace for additional heating in air. After 4 hours the oxidationweight loss was about 1.7 percent. This compares to the oxidation weightloss of about 27 percent for an uncoated graphite disc. After about 24hours the oxidation weight loss was only about 44 percent, whichcompares to the oxidation weight loss of about 100 percent for theuncoated graphite disc. This example demonstrates the surprisinghydrolytic and oxidative stability which is imparted to a graphitesubstrate when coated and subsequently heated in an oxidizingatmosphere.

EXAMPLE 19

The present example demonstrates, among other things, the formation of acoating composition from a mixture of polyureasilazane, zirconium oxideand boron carbide coated onto an aluminum oxide substrate. Example 10was substantially repeated except that the mixture was prepared bycombining about 50 grams of polyureasilazane prepared substantiallyaccording to the method of Example 2, 10 grams of zirconium oxide (0.5μm, SC-15 grade, Magnesium Elektron, Inc., Flemington, N.J.), and 10grams of boron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan,Conn.), and then brush coated onto an aluminum oxide substrate. Thecoated substrate was heated for about 1 hour to about 200° C. in air,and then heated to about 500° C. for about 12 hours in air. The coatedsubstrate demonstrated hydrolytic stability.

EXAMPLE 20

The present example demonstrates, among other things, the formation of acoating composition from a mixture of polytetrafluoroethylene,polyureasilazane, silicon carbide, boron carbide and stainless steelflakes. The coating was prepared by combining about 50 grams ofcomponent A and about 50 grams of component B. Component A was comprisedof about 100 grams of polyureasilazane prepared substantially accordingto the method of Example 2, 35 grams of silicon carbide (E-110, 5 μm,Norton, Liliesand, Norway), 30 grams of boron carbide (B₄ C, 5 μm,ESK-Engineered Ceramics, New Canaan, Conn.), and 30 grams of stainlesssteel flakes (15-40 μm, 316×SS leafing grade, U.S. Bronze Powders, Inc.,Flemington, N.J.). Component B was comprised of about 130 grams ofpolyureasilazane prepared substantially according to the method ofExample 2 and 50 grams of polytetrafluoroethylene (sold under thetrademark Teflon® 7A, DuPont, Wilmington, Del.). The mixture wasairbrushed onto a silicon carbide reinforced aluminum oxide compositebody made substantially in accordance with the teaching of U.S. Pat. No.4,923,832, the subject matter of which is specifically incorporatedherein by reference. After heating, the resulting coated bodydemonstrated good adherence cohesion and scratch resistance.

EXAMPLE 21

The present example demonstrates, among other things, the formation of acoating composition from a mixture of polysilazane, zirconium oxide, andboron carbide. The coating was prepared substantially according toExample 10, except that the composition comprised about 100 grams ofpolysilazane prepared substantially according to the method of Example2, about 30 grams of zirconium oxide (0.5 μm, SC-15 grade, MagnesiumElektron, Inc., Flemington, N.J.), and about 15 grams of boron carbide(B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.). The coatingwas heated to about 850° C. for about 24 hours on a stainless steelsubstrate, and the resulting coated substrate demonstrated good adhesionand cohesion.

EXAMPLE 22

The present example demonstrates, among other things, the formation of acoating composition comprised of multiple layers of differingcompositions. The coating was prepared substantially according to themethod of Example 10, except that the composition of the first layercomprised about 100 grams of polyureasilazane prepared substantiallyaccording the method of Example 2, about 33 grams of stainless steelflake (15-40 μm, 316×SS leafing grade, U.S. Bronze Powders, Inc.,Flemington, N.J.), and about 15 grams of boron carbide (B₄ C, 5 μm,ESK-Engineered Ceramics, New Canaan, Conn.). The second layer comprisedabout 25 grams of polyureasilazane prepared substantially according tothe method of Example 2, about 60 grams of zirconium oxide (0.5 μm,SC-15 Grade, Magnesium Elektron, Inc., Flemington, N.J.), and about 15grams of boron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan,Conn.). The coatings were sprayed onto a mild steel substrate withoutheating between applications. The coated substrate subsequently washeated to about 210° C. for about 1 hour, and then heated to about 850°C. The coating composition was observed to protect the substrate againstheating at about 850° C. for more than about 12 hours.

EXAMPLE 23

The present example demonstrates, among other things, the formation of acoating composition comprising multiple layers of differing compositionson a substrate comprising carbon fiber reinforced AVIMID™ polymercomposite (DuPont, Wilmington, Del.). The coating was preparedsubstantially according to the method of Example 10, except that thecomposition of the first layer comprised about 30 grams ofpolyureasilazane prepared substantially according to the method ofExample 2, about 50 grams of zirconium oxide (0.5 μm, SC-15 grade,Magnesium Elektron, Inc., Flemington, N.J.), and about 20 grams of boroncarbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.). Thesecond layer comprised about 100 grams of polyureasilazane preparedsubstantially according to the method of Example 2, about 40 grams ofzinc, and about 15 grams of boron carbide (B₄ C, 5 μm, ESK-EngineeredCeramics, New Canaan, Conn.). The coating mixtures were applied with anairbrush, and the coated substrate was heated to about 350° C. at about100° C. per hour, and held at about 350° C. for about 15 minutes. Theresulting coating demonstrated good abrasion resistance.

EXAMPLE 24

The present example demonstrates, among other things, the formation of acoating composition comprised of polyureasilazane, strontium boride andstainless steel flake. The coating was prepared substantially accordingto the method of Example 3, except that the composition comprised about90 grams of polyureasilazane prepared substantially according to themethod of Example 2, about 15 grams of strontium hexaboride (40 μm, AlfaProducts, Danvers, Mass.), and about 33 grams of stainless steel flakes(15-40 μm, 316×SS leafing grade, U.S. Bronze Powders, Inc., Flemington,N.J.). The coating mixture was applied with an airbrush, and the coatedsubstrate was heated to about 200° C. for about 1 hour, and then heatedto about 850° C. at a rate of about 500° C. per hour, and held at 850°C. for about 4 hours. The resulting coating demonstrated good adhesion.

EXAMPLE 25

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising polyureasilazane,zirconium oxide and boron carbide. Example 10 was essentially repeatedexcept that the coating mixture comprised about 100 grams ofpolyureasilazane prepared substantially according to the method ofExample 2, about 49 grams of zirconium oxide (0.5 μm, SC-15 grade,Magnesium Elektron, Inc., Flemington, N.J.), and about 21 grams of boroncarbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.). Thecoating was brush coated onto a 316 stainless steel substrate, and thecoated substrate was heated to about 250° C. at a rate of about 100° C.per hour, and held at about 250° C. for about 4 hours. The resultingcoating, which measured about 35 μm thick, was tested under theconditions set forth in ASTM standard D968-81, and demonstrated anabrasion resistance factor of about 32,000.

EXAMPLE 26

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising polyureasilazane,zirconium oxide and boron carbide. Example 25 was essentially repeatedexcept that the coating mixture comprised about 100 grams ofpolyureasilazane prepared substantially according to the method ofExample 2, about 49 grams of zirconium oxide (0.5 μm, MagnesiumElektron, Inc., Flemington, N.J.), and about 21 grams of boron carbide(B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.). The coatingwas brush coated onto a 316 stainless steel substrate, and the coatedsubstrate was heated to about 500° C. at a rate of about 100° C. perhour, and held at about 500° C. for about 4 hours. The resultingsubstrate was tested under the conditions set forth in ASTM standardD968-81, and demonstrated an abrasion resistance factor of about 11,538.

EXAMPLE 27

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising polyureasilazane,zirconium oxide and boron carbide. Example 25 was essentially repeatedexcept that the coating mixture comprised about 100 grams ofpolyureasilazane prepared substantially according to the method ofExample 2, about 21 grams of zirconium oxide (0.5 μm, MagnesiumElektron, Inc., Flemington, N.J.), and about 49 grams of boron carbide(B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.). The coatingwas brush coated onto a 316 stainless steel substrate, and the coatedsubstrate was heated to about 500° C. at a rate of about 100° C. perhour, and held at about 500° C. for about 4 hours. The resultingsubstrate was tested under the conditions set forth in ASTM standardD968-81, and demonstrated an abrasion resistance factor of about 11,428.

EXAMPLE 28

The present example demonstrates, among other things, the formation of acoating composition from a mixture comprising a polyureasilazane,zirconium oxide and boron carbide on both a stainless steel and a carbonsteel substrate. Example 3 was substantially repeated except that thecoating mixture comprised about 25 percent by weight of polyureasilazaneprepared substantially by the method of Example 2, about 60 percent byweight zirconium oxide (0.5 μm, SC-15 grade, Magnesium Elektron, Inc.,Flemington, N.J.), and about 15 percent by weight boron carbide (B₄ C, 5μm, ESK-Engineered Ceramics, New Canaan, Conn.). The substrates,measuring about 30 cm by about 3.8 cm by about 0.4 cm, were brush coatedwith the mixtures, and heated to about 800° C. at a rate of about 200°C. per hour, and held at about 800° C. for about 2 hours. The coatedsubstrates were submerged in a composite melt comprising silicon,aluminum and magnesium having about 30 percent by volume silicon carbideparticulate therein, which was stirred at a rate adequate to distributethe silicon carbide particles substantially homogeneously within themelt, at a temperature of about 750° C. for about 6 hours. The coatedsubstrates were removed from the melt and allowed to cool. Thesolidified composite was easily removed from the coated substrates, andthe substrates remained unabraded, without cracks or spalling.

COMPARATIVE EXAMPLES

The following Examples are directed to compositions which, for reasonsnot fully understood, did not form desirable coatings which wereresistant to certain adverse environments.

Example A

Example 3 was substantially repeated except that the coating mixturecomprised about 50 grams of polyureasilazane prepared substantiallyaccording to Example 2, about 10 grams of nickel boride (˜66 μm, AlfaProduct, Ventron Division, Danvers, Mass.), and about 10 grams ofsilicon carbide (E-110, 5 μm, Norton, Liliesand, Norway). The coatedsubstrate was heated to about 200° C. for about 1 hour in air, and thenheated to about 500° C. for about 12 hours in air. The resulting coatingwas cooled to room temperature and was observed to degrade uponimmersion in water.

Example B

An uncoated graphite disc measuring about 5 cm in diameter by about 0.6cm in height and weighing about 23.02 grams was coated with a mixturesubstantially identical to the mixture prepared in Example 9. The discwas then heated at about 150° C. for about 2 hours in a nitrogenatmosphere, essentially according to Example 9. The coated disc was thenweighed and determined to have a weight of about 23.53 grams. The discwas then heated in a nitrogen atmosphere from room temperature to about850° C. over a period of about 2 hours, held at about 850° C. for about15 minutes and then cooled to room temperature. The coating obtained wasblack in appearance, and the coated disc weighed about 23.30 grams, thusexhibiting a coating weight loss of about 59 percent. The sample wasthen soaked in water at ambient temperature for about 16 hours and driedessentially according to the method described in Example 9. After thedisc was immersed in the water, the water appeared cloudy with apronounced deposit of particulates, and the disc weighed only 23.04grams, thus indicating a weight loss of about 0.27 grams of mass duringwater immersion. Upon removal, the disc had substantially the sameappearance as the uncoated graphite disc originally used to prepare thesample, and the weight of the disc was nearly identical to the weight ofthe uncoated disc (23.02 grams).

Example C

The following example demonstrates a coating prepared with boron oxide.Example A was substantially repeated, except that the coating mixturecomprised about 70 grams of polyureasilazane prepared substantiallyaccording to the method of Example 2, about 10 grams of boron oxide (B₂O₃, crushed, A76-3, Fischer Scientific, Fairlawn, N.J.), and about 20grams of zirconium oxide (0.5 μm, SC-15 grade, Magnesium Elektron, Inc.,Flemington, N.J.). The coated substrate was heated to about 500° C. forabout 1 hour, and then heated to about 800° C. for about 30 minutes. Thecoating was cooled to room temperature and was observed to degrade whenimmersed in water at ambient temperature.

Example D

The following example demonstrates a coating prepared with colloidalsilica. Example A was substantially repeated, except that the coatingmixture comprised about 50 grams of colloidal silicon dioxide (NYACOL®2040, Nyacol Products, Inc., an affiliate of PQ Corp, Ashland, Mass.),about 10 grams of boron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics,New Canaan, Conn.), and about 10 grams of silicon carbide (E-110, 5 μm,Norton, Liliesand, Norway). The coated substrate was heated to about500° C. for about 1 hour, and then heated to about 800° C. for about 30minutes. The coating was cooled to room temperature and was observed todegrade when immersed in water at ambient temperature.

Example E

The following example demonstrates a coating prepared withpolybutylmethacrylate. Example A was substantially repeated except thatthe coating mixture comprised about 50 grams of ELVACITE® (DuPontElvacite, 2045, DuPont, Wilmington, Del.) in xylene, about 10 grams ofboron carbide (B₄ C, 5 μm, ESK-Engineered Ceramics, New Canaan, Conn.),and about 10 grams of silicon carbide (E-110, 5 μm, Norton, Liliesand,Norway). The coated substrate was heated to about 500° C. for about 1hour, and then heated to about 800° C. for about 30 minutes. The coatingbrushed off as a powder.

Example F

The following example demonstrates a coating prepared withpolycarbosilane. Example A was substantially repeated, except that thecoating mixture comprised about 7.5 grams of NICALON® (polycarbosilane,Nippon Carbon Co., Ltd., Dow Corning Corp. (distributor), Midland,Mich.), about 20 ml xylene, about 2.5 grams of boron carbide (B₄ C, 5μm, ESK-Engineered Ceramics, New Canaan, Conn.), and 2.5 grams ofsilicon carbide (E-110, 5 μm, Norton, Liliesand, Norway). The coatedsubstrate was heated to about 500° C. for about 1 hour. The coating wascooled to room temperature and was observed to degrade when immersed inwater at ambient temperature.

Example G

The following example demonstrates a coating prepared withdimethylsiloxane. Example A was substantially repeated except that thecoating mixture comprised about 7.5 grams of dimethylsiloxane (Aldrich14615-3, Milwaukee, Wis.), about 2.5 grams of boron carbide (B₄ C, 5 μm,ESK-Engineered Ceramics, New Canaan. Conn.), and about 2.5 grams ofsilicon carbide (E-110, 5 μm, Norton, Liliesand, Norway). The coatedsubstrate was heated to about 500° C. for about 1 hour. The coating wascooled to room temperature and was observed to degrade when immersed inwater at ambient temperature.

We claim:
 1. A method of forming a coating comprising a first layer andat least one subsequent layer on at least one substrate material,comprising the steps of:1) combining at least one boron source materialcomprising an extended boron-boron bonding network and at least onepolymer comprising at least some silicon-nitrogen bonds to form a firstmixture; 2) providing said first mixture onto at least a portion of atleast one substrate to form a first layer on said at least onesubstrate; 3) combining at least one second boron source materialcomprising an extended boron-boron bonding network and at least onesecond polymer comprising at least some silicon-nitrogen bonds to format least one subsequent mixture; 4) providing said at least onesubsequent mixture over at least a portion of said first layer to format least one subsequent layer on said at least one substrate; 5) heatingsaid at least one substrate material having thereon said first layer andsaid at least one subsequent layer in the presence of at least oneoxidant to cause said coating to form.
 2. The method of claim 1, whereinsaid substrate is heated in the presence of at least one oxidant priorto the application of at least one of said at least one subsequentlayer.
 3. The method of claim 1, wherein said first layer and at leastone of said at least one subsequent layers comprise compositions whichare substantially different from one another.
 4. The method of claim 1,wherein at least one of said first layer and said at least onesubsequent layer further comprises at least one filler.